This invention relates to water treatment methods used during wood panel fabrication processes. More particularly this invention relates to a method and apparatus for filtering and concentrating particulate out of a waste water stream. The resulting concentrate is re-applied to the fabrication process, rather than discarded as waste. The resulting wood panels exhibit less formaldehyde emission during their useful life and stronger internal bonds during testing. The cleaned waste water stream is recycled into the air pollution control system of the fabrication process to collect other particulate. Accordingly, the fabrication process produces improved wood panels with fewer waste by-products.
Wood panel fabrication involves steps of drying the wood product (i.e., furnish, wood flakes, raw material), combining and mixing constituent materials (i.e., wood product, resin, wax and scavenging agents), then pressing the materials into a panel. The output is wood panel (i.e., particle board, plywood, oriented strand board, medium density fiberboard). During the drying operation, by-products including (i) fibers, such as wood ash and wood particles, (ii) semi-solids, including fillers, and (iii) liquids are generated. Such materials are captured by an air pollution control system and collected in a recycling water stream. The liquids and semi-solids include hydrocarbons and other organic molecules. In one known process, by-product gasses are first conditioned with water sprays to achieve adiabatic saturation. As a result, the gas is cooled to the necessary level for hydrocarbon vapors to condense. The fibers, semi-solids and liquids, including hydrocarbon droplets, then are collected by a precipitator and discharged into the recycling water stream.
Removal of the hydrocarbons is especially important when drying resinous species such as Southern Yellow pine or Douglas fir. If not condensed and collected as a droplet, the vapors will condense when discharged to the atmosphere forming a blue haze plume (often characterizing wood drying emissions).
FIG. 1 shows a block diagram of a prior art emission control system 10 used in a wood panel fabrication process. Included is a wet electrostatic precipitator 11, also described in U.S. Pat. No. 4,194,888. The system 10 includes a "first" recycle water quench stage 12 and a "second" clean water flush stage 14. The recycle quench stage 12 includes a preconditioning chamber 24, settling tank 26 and pump 28. The clean water flush stage 14 includes the wet electrostatic precipitator 11, a flush tank 18, and pumps 20, 22.
The precipitator 11 employs a high-intensity ionization electrode configuration which concentrates a charging field in a zone between a disc and a collection tube. The geometry enables formation of a stable, high intensity, electrostatic field. Particle charging of 20 kv per inch is maintained with minimal sparking. An ensuing migration velocity results in a desirably high particulate collection efficiency.
Referring to FIG. 1 and the first stage 12, hot dirty gas, including hydrocarbons, and fibers from the drying wood panel are received into the pre-conditioning chamber 24 at inlet 30. Recycled water from the settling tank 26 is sprayed via pump 28 into the chamber 24 conditioning and cooling the gas stream to achieve adiabatic saturation. Spray nozzles with a large orifice diameter quench the gas stream by producing large diameter droplets. Such large droplets are unlikely to evaporate completely, thereby creating spray-dried particulate. The large nozzles also are less likely to clog. The spent quench water is drained to the settling tank 26 for treatment and recycling. The pre-conditioned gas stream, including hydrocarbon droplets is output at channel 32 into precipitator 11.
The second stage 14 receives the gas stream into a cyclone module 34 of precipitator 11. The cyclone 34 removes any large particles such as wood fibers and large water droplets. Remaining fine mist droplets are transported with the saturated gas as the gas enters precipitator 11. The gas stream passes through distribution devices to insure even flow into collection tubes 36. As the gas enters the tubes 36, the gas passes through a series of high-intensity corona charging fields located at each disk. Within the zone between the disks and the tubes, a high electrostatic charge is imposed on the particles. The droplets and mist together form a water film which flows downward along the length of the tubes 36 aided by gravity and the flow of the gas stream. As the charged particles flow farther down the tubes 36, the electrostatic field forces the charged particles toward the tube walls. Because the fine mist droplets are large relative to the submicron particulate, the droplets are the first particles collected at the tube walls. Clean gas exits the tubes 36 into a clean air plenum, and eventually passes through a fan and up a stack for discharge into the atmosphere.
Periodically clean water from flush tank 18 is sprayed via pump 20 into the precipitator 11 to clean the tubes 36. Because the tubes 36 are continuously wetted during the precipitation and ionization steps, the collection tube surfaces are easily washed by the flush stream. The resulting flush water flows down the tubes 36, is collected in a sump 38 at the bottom of the precipitator 11, then pumped via pump 22 to the settling tank 26.
Typically the solids and particulate accumulate in the settling tank 26, then are removed by surface skimmers, bottom drag chains, aeration/flotation devices, or filters. In practice, the solids removed by these devices can achieve a texture of "pudding". For example, such collected solids typically are 15% to 25% solids by weight. Due to the large percentage of liquid material captured with the solids, waste disposal is difficult. The collected waste typically is disposed of by burning in a wet burner or boiler, or by storage in a landfill. Transporting the waste and storing the waste in a landfill is difficult when the waste includes liquid (i.e., 75% to 85% by weight). Accordingly, there is a need to find clean functional uses for the by-product and to reduce the portions discarded as waste.
Another problem with conventional water treatment systems used for wood panel fabrication processes is that the hydrocarbons tend to stay in solution. Previously, the hydrocarbons have been removed from the water treatment system by disposing of the waste water at rates of several gallons per minute. Simple disposal of such water is undesirable. Further, government regulations for discharging waste water are becoming more restrictive. Accordingly, there is a need to reduce the hydrocarbons in solution within the recycling water stream and a need to achieve higher concentrations of solids, thereby reducing the volume of material and making handling of the material easier.